A system for determining the position of a sensor within a given object and for the display of previously recorded images of the object corresponding to the sensor position has been described earlier by BUCHHOLZ in U.S. Pat. No. 5,383,454. With that system it is also possible to guide the tip of a sensor to a particular location within an object, while the position of the sensor can be observed on a monitor screen which also displays a previously recorded image of that particular region within the object. In that earlier concept, the position of the sensor is determined using a commercially available, three-dimensional sound digitizer.
Another example of an earlier method and appropriate system for the acquisition of diagnostically useful, three-dimensional ultrasound image data has been described by POLZ in the European patent EP 0 736 284 A2. That system incorporates a device by means of which it is possible, by freely and manually guiding the ultrasound scanning head, to assemble from a set of three-dimensional data tomographic images of an entire three-dimensional volume object or space to be examined. The position and orientation of the ultrasound scanning head are registered by an additional, electromagnetic sensor system. The ultrasound scanning head, freely guided by hand by the diagnostician, is preferably provided with a holder which also accommodates the receiver of the said electromagnetic sensor system. The sensor system whose receiver coils pick up magnetic fields emitted by a transmitter, produces sensor output data (both positional and rotational data) which precisely define the spatial position and orientation of the ultrasound scanning head. These are translational X, Y and Z axis data as well as rotational data around these axes.
A prerequisite for sufficiently precise positional and orientational determinations using magnetic field measurements is very detailed information on such extraneous parameters as:
interference fields generated for instance by display monitors, computers or electric motors; PA1 interference patterns produced by highly permeable materials in the magnetic field, for instance metal objects moving within the measuring region; or PA1 electromagnetic interference fields emanating from the AC power supply. PA1 in terms of precise resolution, the system is not affected by external parameters; PA1 the system is easy to handle; even if the positional determination were to be disrupted for instance by an object that strayed in between the acquisition device and the ultrasound scanning head, measurements can continue as soon as a clear view is restored; and PA1 the tracking accuracy is not negatively affected by extraneous electromagnetic fields produced by display monitors and/or electrical equipment. PA1 The principles of optical and photogrammetric positional determination employed in this invention are described, inter alia, in the following textbook: PA1 Jordan/Eggert/Kneissl PA1 Handbuch der Vermessungskunde (manual of geodetic surveying) PA1 10th edition, completely revised PA1 Vol. IIIa/3 PA1 Photogrammetry PA1 J. B. Metzlersche Verlagsbuchhandlung, Stuttgart, 1972 PA1 (see in particular paragraphs 144, 145, 146, 147).